The broad goal of this proposed research is to determine the underlying molecular mechanisms required for S. aureus endocarditis vegetations to develop and to improve diagnosis of endocarditis by the use of state-of-the-art molecular imaging techniques. Acute bacterial endocarditis (ABE) is a potentially deadly disease caused by bacterial infection of heart valves. Coagulase-positive S. aureus infections are the leading cause of ABE and have the highest mortality rates, due to the increased incidence of antibiotic-resistance. 5. aureus expresses a unique non-proteolytic prothrombin (ProT) activator, staphylocoagulase that mediates recognition of fibrinogen by the active staphylocoagulase-ProT complex resulting in conversion of fibrinogen to fibrin. Fibrin generation is hypothesized to play a critical role in ABE pathology. During the K99 phase of my project, we developed novel active site-labeled ProT analogs that were use in Fluorescence Molecular Tomography (FMT) and Positron Emission Tomography fused with Computed Tomography (PETCT) studies to identify 5. aureus ABE, non-invasively in a mouse model ofthe disease. We discovered that staphylocoagulase not only activates ProT, but also localizes the staphylocoagulase- ProT complex to S. aureus vegetations by COOH-terminal interactions with Fragment D of fibrin/fibrinogen. Here, my newly established lab will use these imaging agents to assess whether staphylocoagualse is a virulence factor for 5. aureus in endocarditis. Furthermore, specific recognition of vegetations by our new endocarditis probes would aid in determining the role and importance of factor Xllla cross-linking to the pathology of endocarditis and facilitate identification of new adjunctive therapies targeting fibrin generation. Molecular imaging, biochemical, and immunology studies are proposed to test our hypotheses: that these SC is a virulence factor;that factor XIII activation stabilizes the fibrin network during ABE;and that new adjunctive therapies can limit vegetation grow in ABE. Specific Aims are: (1) To determine whether staphylocoagulase is a virulence factor for S. aureus endocarditis (2) To determine whether S. aureus-induces activation of FXIII and fibrin cross-linking during ABE;and (3) To characterize the effect of argatroban and staphylocoagulase inhibitors on the vegetation development during ABE.